1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a plurality of power generation units. Each of the power generation units comprises first and second electrolyte electrode assemblies, and is formed by stacking the first electrolyte electrode assembly on a first separator, a second separator on the first electrolyte electrode assembly, the second electrolyte electrode assembly on the second separator, and a third separator on the second electrolyte electrode assembly. Each of the first and second electrolyte electrode assemblies includes a pair of electrodes and an electrolyte interposed between the electrodes. A reactant gas passage for at least a fuel gas or an oxygen-containing gas as one of reactant gases extends through the power generation units, and a coolant flow field for a coolant is formed between the power generation units.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs an electrolyte membrane. The electrolyte membrane is a polymer ion exchange membrane, and interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly and the separators make up a unit cell for generating electricity. In use, typically, a predetermined number of unit cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas flow field for supplying a fuel gas is formed on a separator surface facing the anode, and an oxygen-containing gas flow field for supplying an oxygen-containing gas is formed on a separator surface facing the cathode. Further, a coolant flow field is formed between the separators for supplying a coolant along the surfaces of the separators.
In order to reduce the length of the fuel cell stack in the stacking direction and reduce the weight of the fuel cell stack, so far, attempts to reduce the number of the coolant flow fields have been made by adopting so called skip cooling structure where the coolant flow field is provided for every predetermined number of unit cells.
For example, as shown in FIG. 11, Japanese Laid-Open Patent Publication No. 10-189011 discloses a fuel cell formed by stacking a plurality of cell units 3, and each of the cell units 3 is formed by stacking a first separator 1a, a first MEA (membrane electrode assembly) 2a, a second separator 1b, a second MEA 2b, and a third separator 1c. 
Each of the first and second MEAs 2a, 2b includes an ion exchange membrane 4a, and an anode 4b and a cathode 4c fixed on both surfaces of the ion exchange membrane 4a. A fuel channel forming member 5a is provided on a surface of the first separator 1a facing the anode 4b of the first MEA 2a, and a fuel supply channel 5b is formed in the fuel channel forming member 5a. A coolant water channel forming member 6a is provided on the other surface of the first separator 1a, and a coolant water supply channel 6b is formed in the coolant water channel forming member 6a. 
An oxygen channel forming member 7a is provided on a surface of the second separator 1b facing the cathode 4c of the first MEA 2a, and an oxygen supply channel 7b is formed in the oxygen channel forming member 7a. A fuel channel forming member 8a is provided on a surface of the second separator 1b facing the anode 4b of the second MEA 2b, and a fuel supply channel 8b is formed in this fuel channel forming member 8a. 
An oxygen channel forming member 9a is provided on a surface of the third separator 1c facing the cathode 4c of the second MEA 2b, and an oxygen supply channel 9b is formed in the oxygen channel forming member 9a. 
Though not shown, in the fuel cell, supply passages and discharge passages extend through the first separator 1a to the third separator 1c for supplying and discharging the fuel, the oxygen, and the coolant water.
For example, the fuel is supplied to inlets of the fuel supply channels 5b, 8b through the respective supply passages, and the fuel is discharged into the discharge passages from outlets of the fuel supply channels 5b, 8b. Likewise, the oxygen is supplied from the common passages to the oxygen supply channels 7b, 9b, and the oxygen is discharged into the discharge passages from the outlets of the oxygen supply channels 7b, 9b. 
In this case, for example, the fuel inlets and the fuel outlets are provided individually corresponding to the fuel supply channels 5b, 8b. For simplification of the flow field structure, it is desirable that the fuel supply channels 5b, 8b have common flow field portion.
However, for example, in the fuel outlet, if the outlet side of the fuel supply channel 5b is closed by the water produced in the power generation, the exhaust gas from the fuel supply channel 8b flows preferentially in the common flow field near the discharge passage. Therefore, the produced water cannot be discharged from the fuel supply channel 5b, and the performance may be degraded due to the insufficient stoichiometric ratio.